February 4. Archaeocyathids

Part of the early Cambrian explosion included the development of the archaeocyathids, somewhat conical animals with calcareous skeletons rather like corals. But they are not corals. Their affiliations are not 100% certain, and some scientists put them in their own phylum, but I think most taxonomists are classifying them as early sponges.

Their name means “ancient cup” and some are long inverted cone shapes as much as 30 centimeters long, close to 12 inches, while others are like nested bowls. Some were solitary, but others grew together to make some of the earliest significant reefs on earth.

They appeared about 525 million years ago, part of a famous fossil assemblage called the Tommotian fauna for the specimens found along the Tommot River in Siberia. They’ve been found all over the world, and they are distinctive enough that they serve as index fossils for the early Cambrian. That means rocks can be correlated across large distances using the particular varieties of archaeocyathids that are found in them.

Just nine or ten million years after they appeared and proliferated, the archaeocyathids went into a sharp decline in numbers and diversity about 516 million years ago. Interestingly, this time is when more modern sponges began a rapid diversification, another pulse in the Cambrian explosion. The sponges that evolved then have survived with variations to this day, but the archaeocyathids were all extinct before the end of the Cambrian.

—Richard I. Gibson

Drawing of reconstructed archaeocyathids by Stanton F. Fink, via Wikipedia under GNU free documentation license.  

February 3. The Cambrian Explosion

O Wonder! How many goodly creatures are there here! —Shakespeare



The Cambrian Explosion was not a violent volcanic eruption, but rather the sudden appearance in the fossil record of abundant and diverse shelly fossils, the remains of relatively large animal life. Even Charles Darwin recognized it, and saw it as an important objection to his theory of evolution by natural selection.

Like most geologic events the Cambrian explosion wasn’t instantaneous, but it did occur over a geologically short time, around 542 to 530 million years ago, right at the start of the Cambrian period of the Paleozoic Era.

Today’s podcast is a discussion between me and geologist Dr. Colleen Elliot. Neither of us is an expert on paleontology, so this will be something of an exploration for all of us.

Here are some definitions of terms non-geologists may be unfamiliar with in the discussion. Phanerozoic is the name of the eon, the largest subdivision of geologic time, that starts with the Cambrian period of the Paleozoic era. We talked about that a bit the other day. And the Great Unconformity is a break in the rock record that is nearly global in extent, a time before the first rocks of the Cambrian were deposited. It represents a time of near-global erosion of the older Precambrian rocks, and it means that the continents were standing above sea level so that they could be eroded.

—Richard I. Gibson

Links for further reading:
McKenzie et al. on climate and evolution (January 2014)
http://www.sciencemag.org/content/341/6152/1355
Smith and Harper on summarizing Cambrian explosion causes (Sept. 2013)
http://www.astrobio.net/pressrelease/5758/oxygen-not-the-cause-of-the-cambrian-explosion
https://www.pbs.org/wgbh/evolution/library/03/4/l_034_02.html
http://ucrtoday.ucr.edu/9063

February 2. Cambrian time scale

Almost from the start, geologists have argued, sometimes passionately, about exactly where the divisions of geologic time should begin and end. In the early days, when all ages were relative, meaning this is older than that, but we didn’t know absolute age dates closer than tens of millions of years or more, it was challenging to assign a particular time to the start of a period like the Cambrian. When I was in college, the start of the Cambrian was put at 600 million years ago, and honestly, it was a fairly arbitrary number even though it was generally accepted. My historical geology textbook, published in 1960, has a question mark after 600 million years.

Much of geologic research is devoted to pinning down the details of ideas and concepts that were sketched out in the early 1800s. So with better age dates and much more information from the far corners of the world, the timing of the start of the Cambrian has been refined over time. When I wrote the history of the earth calendar in 1994, the generally accepted start of the Cambrian was about 575 million years ago. Today that date is 542 or 543 million years ago, but even now that’s not universally accepted and the USGS time scale here has slightly different dates. It isn’t that the time changes, but our way of looking at the data does change, and everyone doesn't quite agree on all the details.

The Cambrian Period today is considered to have lasted until about 488 million years ago, for a total length of 54 million years, just 1% of Earth’s history. Yet we’re spending all of February on the Cambrian. Stuff happened!

Like the months of the year, the periods of earth history are subdivided and given names to make it easier to refer to them. So we have Early, Middle, and Late Cambrian as general names, and in parts of North America those times have been related to packages of rocks called the Waucoban Series, Albertan Series, and Croixan Series. You can imagine that everything geologic didn’t happen simultaneously, or in the same way, all around the world, so there are dozens of regional variations in the nomenclature. The subdivisions are not arbitrary, but are often based on the fossils that can be found in each subset of the rocks. Fossils can be related to age, and can help correlate between rocks of different type that may be of the same age.

There are ongoing attempts to create internationally acceptable time scales. So there is actually an International Subcommission on Cambrian Stratigraphy that works on this kind of thing, and at present the Cambrian is divided into four major subdivisions, based in part on the timing of major extinction events. I think we’ll settle for early, middle, and late.

—Richard Gibson

Further reading:

http://tapestry.usgs.gov/ages/ages.html
International Commission on Stratigraphy
http://en.wikipedia.org/wiki/Cambrian